Light-emitting diodes (LEDs) have become a symbol of energy-saving industry owing to their low power consumption and long operation life. Meanwhile, LEDs are generally expected to replace traditional fluorescent illumination devices. Whether the LEDs can be broadly implemented in a variety of lighting apparatus or not depends on their light-emitting efficiency and cost. Therefore, how to increase the light extraction efficiency becomes one of the decisive factors to improve the light-emitting efficiency.
Surface roughening has been an effective means to enhance the light-extraction efficiency, whether on a substrate or on a top semiconductor layer. In the U.S. Pat. No. 6,091,085, as shown in FIG. 1, Lester discloses a light-emitting device having a surface-roughened substrate. LED 10 comprises a sapphire substrate 11 and a plurality of protrusions 111 and depressions 112 formed therein for scattering light emitted from a light-emitting layer 14. Therefore the light extraction efficiency of the light-emitting device is enhanced. The plurality of protrusions 111 and depressions 112 are formed by randomly roughening the surface of the substrate 11 by mechanically polishing or reactive-ion-etching (RIE). Although the light extraction efficiency is enhanced because of the roughened surface of the substrate 11, the surface damages such as the alteration of the surface composition and the lattice distortion are resulted from the roughening process as well. Besides, part of the mask materials or the reactive ions are sputtered onto and even doped into a certain depth of the substrate, therefore the quality of the epitaxy layer deposited thereafter is downgraded. The mechanical polishing process also damages the lattice structure of the substrate surface, such that the quality of the successively grown semiconductor layer, such GaN, is worse, and the internal quantum efficiency is lowered.
Wang et al. published a paper in Journal of The Electrochemical Society, 153(3) C182-185 studying a maskless method for Epitaxial Lateral Overgrowth(ELOG) to grow GaN layer. The purpose of the paper is to simplify the complexity of the traditional ELOG process using SiO2 as a hard mark. The study focuses on growing a GaN film having a lattice structure with lower defect density on a sapphire substrate, especially with lower threading dislocation defects (TDDs) caused by the lattice mismatch between the sapphire substrate and the GaN film. Wet etching scheme is utilized to form plural trenches of V shape or U shape, and further correlate the influence of the exposed facets of the sapphire substrate formed under different etching conditions upon the TDD density of subsequently-formed ELOG GaN film.